The present invention relates to a method and apparatus for determining the heating value and the relative density (or specific gravity) of a hydrocarbon fuel. These values are used to define the energy content and/or the Wobbe Index of a hydrocarbon fuel.
Contracts for the supply of hydrocarbon fuels are based upon the average values of the energy content of the gas on an hourly or daily basis. Accordingly, the primary concern of designers or equipment for the measurement of the energy content of fuels has been on the accuracy of the measurements. The most commonly used instrument today to measure the energy content of fuel gases for industrial use, is the process gas chromatograph (PGC). This instrument, sometimes referred to as ‘BTU analyser’, is characterised by having a high accuracy, but also a long analysis time, of the order of several minutes, typically from 3 to 5 minutes. Further, the instrument is complex to construct, operate and maintain and requires multi-component gases in order to calibrate. These factors provide the need for an improved means for measuring the energy content of a fuel that is both fast and accurate, while being simple to construct, operate and maintain.
The Wobbe Index is a calculated value used to provide a comparison of the combustion energy output of different fuel streams, in particular gaseous fuels. By comparing the Wobbe Index of different fuels, an indication of their different energy outputs during combustion can be obtained. Thus, two fuels of different composition having a similar Wobbe Index will provide a similar energy output when combusted under a given set of conditions.
The Wobbe Index (WI) is calculated from the heating value (HV) and the relative density (RD) of a fuel and is defined by the following formula:WI=HV/√{square root over (RD)}.
The Wobbe Index is used on a commercial scale to compare the combustion energy output of different hydrocarbon fuels, in particular hydrocarbon fuel gases. Accordingly, the accurate determination of the Wobbe Index is important for a wide range of industries, most notably the natural gas industry.
There is a particular need for a means of determining the Wobbe Index that is both fast and accurate for application in fuel burner installations, especially gas burner installations. This need arises as a result of safety and environmental issues in connection with the operation of the burner installation. Such an improved means for determining the Wobbe Index would also be of considerable use in fuel mixing stations, where a fast analysis time is required in order to reduce the internal volume of the buffer lines employed in the fuel supply system.
Fast meters for determining the energy and/or the Wobbe Index are known. A first type is based on the requirements of ASTM Standard D-4891, entitled ‘Standard Test Method for Heating Value of Gases in Natural Gas Range by Stoichiometric Combustion’. D-4891 concerns the determination of the heating values of gases within a certain composition range. The method is based upon the linear relationship between the heating value of natural gas and the stoichiometric gas oxidation flow disclosed in the standard. The fuel is combusted with oxygen (or air) and either the temperature of the combustion or the oxygen (or air) flow rate at the outlet is measured in order to determine the stoichiometric oxidation gas flow conditions. The method disclosed in D-4891 is fast, having a response time of less than 15 seconds, and is relatively accurate, with an accuracy of about 0.4%. The method also has the advantage that it can be applied to a wide range of gas compositions without the need for extensive and time consuming recalibration. However, in order to determine the Wobbe Index, the method of D-4891 must be combined with a method for measuring the density of the fuel gas having the same level of accuracy and speed of response.
A second type of fast meter for determining the energy content and/or the Wobbe Index of a fuel is applied to the measurement of so-called generic fuel gases, that is natural gas produced as a result of hydrocarbon exploration and production activities. The composition of generic natural gases is assumed to consist of four dependent components: nitrogen (N2), carbon dioxide (CO2), methane (CH4) and the so-called ‘equivalent hydrocarbon’ (CH). The equivalent hydrocarbon represents the higher hydrocarbons of the gas, which are assumed to consist essentially of alkanes. According to the one third rule generally applicable to generic gases, the molar ratio of consecutive higher hydrocarbons is about one third, that is, for example, the molar ratio of propane to ethane is about ⅓, the molar ratio of propane to butane in the gas is about ⅓, and so on. These characteristics allow generic natural gases to be characterised using three independent properties of the gas, referred to as the ‘correlative principle for generic natural gases’. Meters for the determination of the energy content and/or the Wobbe Index for generic natural gases operating on the principle of the one third rule are referred to as “Correlative Meters” and apply the measurement of three independent properties. Examples of suitable independent properties include the heating value of the gas, the density of the gas, the velocity of sound in the gas, the heat capacity of the gas, the heat conductivity of the gas and the concentration of carbon dioxide. Such meters are characterised by combining a fast response time, typically less than 15 seconds, with an acceptable accuracy of measurement of both the heating value and density of the gas, typically within 0.5%. However, as a result of the principles on which these meters operate, in particular the equivalent hydrocarbon theory and the one third rule, their application is limited to naturally produced fuel gases. Correlative meters cannot be applied to other fuel gases, such as natural gas that has been treated, for example to strip propane and butane, or blended with nitrogen, and other fuel gases, such as biogas, refinery gases and the like, since such fuels do not have a composition that follows the one third rule.
Accordingly, there is a need for a means for determining the energy and/or the Wobbe Index of a wide range of fuels, not just natural gases, that combines a high speed of response and a high degree of accuracy. It would also be advantageous if such means could be simple to construct and operate, requiring little or no calibration and little or no maintenance.